2.1. Metabolic and Regulatory Roles of Glutamate in Plants
Glutamate has important roles in plant nitrogen metabolism. Glutamate is the amino acid into which inorganic nitrogen is first assimilated into organic form. Plants have three distinct nitrogen processes related to nitrogen metabolism: (1) primary nitrogen-assimilation, (2) photorespiration, and (3) nitrogen "recycling." All three processes involve assimilation of ammonia into glutamate and glutamine by the operation of glutamine synthetase (GS) and glutamate synthase (GOGAT). Glutamate and glutamine, being the first products of nitrogen-assimilation, in turn serve as nitrogen donors in the biosynthesis of essentially all amino acids, nucleic acids, and other nitrogen-containing compounds such as chlorophyll (Lea et al., in: Recent Advances in Phytochemistry, edited by Poulton et al., New York and London: Plenum Press, 1988, pp. 157-189).
Glutamate is also a principal "nitrogen-transport" compound in plants. It and glutamine are two of only four amino acids used to transport nitrogen within a plant (Lea and Miflin, in: The Biochemistry of Plants, Vol. 5, edited by Stumpf and Conn, Academic Press, 1980, pp. 569-607; Urquhart and Joy, 1981, Plant Physiol. 68:750-754). In light-grown metabolically active plants, glutamate and glutamine are used in anabolic reactions and are transported as such. By contrast, in etiolated or dark-adapted plants, glutamine is converted into inert asparagine for long-term nitrogen storage.
Glutamate also may be a signal or regulatory molecule in regulating the expression of plant genes. Specifically, glutamate along with glutamine and asparagine appears to have an antagonistic role to that of sucrose in regulating certain nitrogen assimilation genes. Sucrose has been shown to induce the expression of genes for nitrate reductase (NR), nitrite reductase (NiR), and chloroplastic glutamine synthetase (GS2) in tobacco (Saur et al., 1987, Z. Naturforsch. 42:270-278; Vincentz et al., 1993, Plant J. 3:315-324). Sucrose also induces genes for GS2 and ferroredoxin-dependent glutamate synthase (Fd-GOGAT) in Arabidopsis. Sucrose-induction of the NR and NiR in tobacco is suppressed by subsequent additions of glutamine, glutamate or asparagine to the media (Vincentz et al., ibid.). Conversely, a nitrogen metabolism gene, glutamine-dependent asparagine synthetase (ASN1), in Arabidopsis is repressed by light or sucrose (Lam et al., 1994, Plant Physiol. 106:1347-1357). The sucrose repression of ASN1 can be relieved by additions of glutamine, glutamate, or asparagine (Id.). Sucrose induction of GS2 genes by asparagine and glutamine has been demonstrated in lupine embryos (Ratajcak, et al., 1981 Physiol. Plant 51:277-280).